In IMT2000-TDD (International Mobile Telecommunications 2000—Time Division Duplex) system, a midamble which is a pilot series is inserted in the center of a time slot of a communicated signal as shown in FIG. 1A and FIG. 1B. In this system, one time slot is the data modulation unit. Moreover, data is not necessarily transmitted in the next time slot, therefore a burst transmission is performed.
A midamble to be inserted into the center of the time slot is generated by cycling a reference code (basic midamble code) by a predetermined number of chips on transmitting side, and plural kinds of midambles are generated.
A method of generating midambles will be explained with reference to FIG. 2. Midamble 1001-1 to midamble 1001-8 each have a length of 144 chips, and these midambles for 8 users (8 kinds of midambles) are generated by cyclic-shifts by 16 chips in the range where two basic midambles 1002 that are each a known series consisting of 128 chips are combined. These midambles are sometimes called midamble shifts. One of the 8 kinds of midamble shifts is assigned to each user signal and transmitted.
As a method of assigning midamble shifts to users, there are three schemes: “default midamble assigning scheme” in which the relationships between used spreading codes and midamble codes are decided beforehand; “user specific midamble assigning scheme,” in which each user is assigned an individual midamble code; and “common midamble assigning scheme,” in which all users are assigned a common midamble code. FIG. 1A shows the slot structure of a transmit signal in the default midamble assigning scheme. FIG. 1B shows the slot structure of a transmit signal in the common midamble assigning scheme.
The receiving side, when calculating a midamble correlation, performs a correlation calculation by cyclically shifting the basic midamble code each time by 16 chips, to generate delay profiles corresponding respectively to the midamble 1001-1 to midamble 1001-8 based on the calculation results. Then some paths are selected based on the generated delay profiles, and RAKE combination is performed on the selected paths. A JD (Joint Detection) operation is performed using a result of the RAKE combination and the delay profiles.
In case the transmitting side performs discontinuous transmission (DTX), the data transmission is suspended if data to be transmitted runs out during transmission.
However, with conventional apparatuses, there is the problem that, when the common midamble assigning scheme is used on the receiving apparatus side, the apparatus can not determine if data is being transmitted to the apparatus by observing the midamble. At this time, power is wasted in processing such as despreading, RAKE combination, and JD operation. There is also the following problem. That is, if the apparatus executes averaging the delay profiles over time in path-search even though no signal is sent to the apparatus, the accuracy in estimating path position degrades. Moreover, the gain of an AGC (Auto Gain Control) is set at a meaningless value.